Crystalline explosives in a viscoelastic binder of sheet form



Jan. 4, 1966 w. JONES ETAL 3,227,588

CRYSTALLINE EXPLOSIVES IN A VISCO-ELASTIC BINDER OF SHEET FORM Filed March 11, 1964 mmfi $52 Zfi 028E002. 52; $36 5:5 J? k 55E 5E; mzmoixu u moimommouz United States Patent 3,227,588 CRYSTALLINE EXPLOSIVES IN A VISCO- ELASTIC BINDER 0F SHEET FORM Walter Thomas Jones, Petts Wood, Kent, and John Wilby, Eltham, London, England, assignors to Her Britannic Majestys Principal Secretary of State for the War Department, London, England Filed Mar. 11, 1964, Ser. No. 351,138 Claims priority, application Great Britain, Mar. 14, 1963, 10,115/63 Claims. (Cl. 14918) The present invention relates to explosive compositions that are particularly useful for the production of explosive sheets, which are used, for example, to harden metal surfaces and to form, shape or demolish metal articles.

Explosive compositions in accordance with the present invention provide explosive sheets that have suflicient mechanical strength to retain an imposed shape without support and suflicient flexibility to be bent through acute angles without materially cracking or breaking. Furthermore, these explosive sheets do not require coating either to provide strength or to remove surface stickiness.

In accordance with the present invention, an explosive composition that is suitable for the production of explosive sheets comprises between 85% and 90% by weight of a crystalline high explosive, such as R.D.X. (cyclo trimethylene trinitramine), H.M.X. (cyclo tetramethylene tetranitramine), P.E.T.N. (pentaerythritol tetranitrate) or tetryl (2,4,6-trinitrophenylmethylnitramine), in a fine particulate form with a bimodal particle size distribution bound 7 in a visco-elastic binder of which between about 3 and 25% by weight is a highly fluorinated polymeric halocarbon. The term highly fluorinated throughout the specification and claims includes fully fluorinated. If there is less than 85% by weight of the explosive there is an increasing chance (particularly for thin sheets) that the sheet will not propagate a detonation. If there is more than 90% of explosive the sheet is brittle and lacks mechanical strength. The proportion of explosive is preferably between 87% and 89% by weight.

To produce an explosive composition containing these high proportions of explosive powder and yet having suitable rheological properties, the explosive must be I present as a fine powder (i.e. at least 95% of the particles are less than 100 microns diameter) which has a bimodal particle size distribution so that a high degree of particle packing can be achieved with separation of the particles in the binder matrix. The measured particle size distribution of a suitable explosive powder showed one peak between and microns and another between 8 and 10 microns while practically all of the powder had a particle size of less than about 50 microns.

The visco-elastic binder used in the explosive composition is a Bingham material whose response to stress conforms to the superposition of elements which obey Hookes elastic law and elements which obey Newtons viscosity law. Visco-elastic substances which are preferably used as the main constituent (i.e. comprising more than 50% by weight of the viscoelastic binder) are aliphatic hydrocarbon polymers, such as polyisobutylene or polypropylene, having a substantially linear polymer chain with lower alkyl side-groups. Polyisobutylene is a satisfactory binder when its molecular weight is at least of the order of 5,000 (when measured by S'taudingers viscosity method); too low a molecular weight results in a sticky explosive composition that lacks mechanical strength. When the binder has a relatively high molecular weight it may be too viscous for ready incorporation with the explosive even in a water slurry, in which case the viscosity may be reduced by blending with a suitable amount of either a lower molecular weight polymer or a plasticiser. For example, it has been found that polyisobutylene with a molecular weight of about 15,000 (Staudinger) requires the addition of about 20% of a plasticiser before it can be readily incorporated with the explosive.

Suitable plasticisers are aliphatic esters containing at least 15 carbon atoms and the esters of dibasic acids, such as sebacic acid and phthalic acid,.are particularly suitable. Examples of suitable plasticisers include ethyl oleate, dibutyl phthalate and di(2-ethylhexyl) sebacate.

The presence of extremely fine particles (i.e. particles of less than 1 micron in diameter) of a polymeric highly fluorinated halocarbon in the binder of the explosive composition, confers the ability on explosive sheets made of the composition to remain in contact with a surface without having excessive surface stickiness and generally enhances their mechanical and flexing properties. In particular, the resistance to tearing is increased and the chances of cracking during flexing are reduced.

Suitable fluoro-poly-mers are aliphatic polymeric fluorocarbons that do not contain any hydrogen but may contain another halogen, for example, chlorine and which are made by a suspension process so that they can separate in extremely fine particles in the composition. Such compounds are exemplified by polytetrafluoroethylene, polyhexafluoropropylene and polytrifluorochloroethylene.

The preferred fluoropolymer is polytetrafluoroethylene (P.T.F.E.), which is available commercially (for example, grade CD3 supplied by Messrs. I.C.I. Ltd.) as a powder consisting of aggregates (about 500 to 600 microns in diameter) composed of individual particles, each about 0.1 micron diameter, so that the aggregates can be broken down in the final manufactured explosive composition.

If the fluoropolymer is present in only a small proportion (at least 3%) of the visco-elastic binder there is some improvement in the properties of the explosive composition but proportions between 5 and 20% by weight, and preferably between 7 and 15% by weight must be added to obtain the best-results.

There is no overall improvement in the properties of the composition as the proportion of fluoropolymer is increased beyond 20% by Weight of the binder, and as the mixing and rolling of the composition in manufacture becomes increasingly ditlicult as the proportion increases beyond 20%, the proportion should not exceed 25%.

Furthermore, the fluoropolymer stifi'ens the composition so allowance should be made for this when determining the viscosity of the binder, especially for additions of more than 10% say. A suitable overall viscosity value for the binder with the fluoropolymelr added is between 40,000 and 200,000 poises determined 'by the extrusion method using Mooneys equation at a rate of shear of 5sec. at25 C. V

Crystalline high explosives are not produced in the form of fine particles as required for the explosive composition and any treatment such as crushing the neat high explosive to produce fine particles is preferably avoided. However, in accordance with a feature of the invention, an explosive composition as hereinbefore described may be readily manufactured by forming'a mixed composition of the constituents in the required proportion and then repeatedly passing the mixture through differential rollers having a separation which is less than 0.01 inch, and is preferably about 0.002 to 0.003 inch.

In accordance with the invention, therefore, a process for the manufacture of an explosive composition comprises intermixing a composition containing to by weight of a crystalline high explosive in a visco-elastic =3 passing the mixed composition through differential rollers having a separation less than 0.01 inch, whereby the high explosive is ground into fine particles having a bimodal size distribution and the polymeric halocarbon is dispersed in the binder.

It is generally impractical, especially in large scale processes, to incorporate the explosive with the binder after the fluoropolymer has been added, and the explosive is then incorporated with the plasticized binder before the fluoropolymer is added. In practice a Water slurry of the high explosive is intermixed with the binder which is then drained before the addition of the fiuoropolymer.

The differential rollers in practice should be knurled as otherwise it is difiicult to pass the moist composition through the narrow gap. The number of passes required to grind the explosive particles to the required size is reduced by decreasing the gap between the rollers and by increasing the difference between the peripheral speeds of the rollers which causes shearing of the particles. Between and 30 passes should be suficient if a gap of about 0.002 to 0.003 inch is used with a differential peripheral speed of the rollers of the order of three feet/sec. This differential rolling also breaks down the aggregates of fluoropolymer particles and disperses the particles through the composition.

The explosive composition is then dried and can be formed into an explosive sheet simply by a final rolling between rollers separated by the desired thickness of the sheet which may in practice be down to about 0.08 inch.

Explosive compositions in accordance with the invention are essentially as follows:

Percent High explosive 85-90 Viscose-elastic binder 9.5l2

Fluoropolymer 0.5-3

By way of example, a typical general explosive composition is:

Percent High explosive 87-89 Polyisobutylene+plasticiser 10.5-11 P.T.F.E. 11.5

The high explosive may be R.D.X., I-LM.X., P.E.T.N. or tetryl.

A specific composition A which falls within this general composition is as follows:

Percent R.D.X. 88.0 Polyisobutylene (Oppanol B15) 8.4 Di(2-ethylhexyl) sebacate 2.4 P.T.F.E. 1.2

An alternative composition B using a less viscous grade All the above percentage proportions are by weight.

By way of example, the manufacture of an explosive composition having the specific composition A and the production of explosive sheets therefrom will now be described. The complete manufacture is illustrated by the accompanying diagrammatic drawing.

Polyisobutylene (70 parts by weight) is stirred and heated in an incorporator to a temperature between about 90 and 100 C. Di(2-ethylhexyl) sebacate parts by weight) is added and the mixture stirred until a mix of uniform composition is obtained. A slurry of R.D.X. 88 lb.) and water (500 lb.) is heated to a temperature of about 95 C. and stirred at 200 r.p.m. in a mixing vessel. The plasticised polyisobutylene (10.8 lb.) is

added and stirring continued for one hour, at the end of which time the particles of R.D.X. are uniformly coated. The suspension is fiitered through a oambric filter to remove excess Water and the filtered material containing about 20% of water is thoroughly mixed with P.T.F.E. (1.2 lb.) in a Werner Pfleiderer incorporator in the cold for one hour. The explosive composition is then passed repeatedly through a difierential rolling machine having knurled rollers rotating at different peripheral speeds so that there is a pronounced shearing action on the composition as it .passes between the nip between the rollers.

A suitable machine is a Torrance drum feed roll set to a minimum separation between its knurled rollers of 0.003 inch and having a 12 in. diameter roll rotating at about 15 r.p.m. and a 15 in. diameter roll rotating at about 60 r.p.m. The peripheral speeds of the two rolls are thus about 50 and 250 ft./min. giving a differential speed of about 200 ft./min. Under these conditions, the composition is repeatedly passed through the rolls by the circulating drum feed for about five minutes, this being theoretically equivalent to about 25 passes.

The explosive composition is then finally dried in any suitable drier. A Werner Pfieiderer incorporator may be used, for example, treatment lasting for about four hours at 95 C. or the composition can be left as thin sheets and simply dried in an oven.

As the binder composition is made more viscous by the addition of the P.T.F.E. it is easier to incorporate the R.D.X. before mixing in the P.T.F.E., in the way already described. However, on a smaller scale (about 2 kg.) incorporation of R.D.X. with the complete visco-elastic binder composition containing the P.T.F.E. is practicable. The differential rolling machine may then be a Torrance Laboratory Roll through which the explosive composition is given ten passes with a minimum separation of about 0.003 inch.

The explosive sheets are made by passing the dried explosive composition through a rolling machine, having two smooth steel rollers of the same diameter, rotating at identical speeds, with the gap set to give the desired sheet thickness down to about 0.08 inch.

Vie claim:

1. A flexible explosive sheet composition which comprises between and by weight of a crystalline high explosive in a fine particulate form with a bimodal particle size distribution which is bound in a visco-elastic binder of which between 3 and 25% by weight consists of extremely fine solid particles of a highly fluorinated polymeric halocarbon having a diameter less than 1 micron through said binder.

2. A flexible explosive sheet composition according to claim 1 in which said crystalline high explosive is a nitrarnine explosive.

3. A flexible explosive sheet composition according to claim 1 in which the said polymeric halocarbon is polytetrafiuoroethylene.

d. A flexible explosive sheet composition according to claim 3 in which the polytetrafiuoroethylene is present in a proportion between 5 and 20% by Weight of the viscoelastic binder.

5. A flexible explosive sheet composition according to claim 4 in which the polytetrafluoroethylene is present as particles of the order of 0.1 micron diameter in a proportion between 7 and 15% by weight of the visco-elastic binder.

6. A flexible explosive sheet composition according to claim 1 in which the high explosive is cyclo trimethylene trinitramine.

7. A flexible explosive sheet composition according to claim 1 in which the high explosive is cyclo tetramethylene tetranitramine.

8. A flexible explosive sheet composition according to claim 1 in which the high explosive is pentaerythritol tetranitrate.

9. A flexible explosive sheet composition according to claim 1 in which the high explosive is 2,4,6-trinitrophenylmethylnitramine.

10. A flexible explosive sheet composition which comprises between 85 and 90% by weight of a crystalline high explosive in a fine particulate form having a bimodal particle size distribution which is bound in a visco-elastic binder comprising at least 50% by weight of an aliphatic hydrocarbon polymer having lower alkyl side chains, between 3 and 25% by weight of extremely fine solid particles of a highly fluorinated polymeric halocarbon having a diameter less than 1 micron, and a plasticizer for the said hydrocarbon polymer.

11. A flexible explosive sheet composition according to claim in which the said hydrocarbon polymer is polyisobutylene.

12. A flexible explosive sheet composition which comprises between 85 and 90% by weight of a crystalline high explosive in a fine particulate form having a bimodal particle size distribution which is bound in a visco-elastic binder comprising at least 50% by weight of polyisobutylene, between 3 and 25% by weight of extremely fine solid particles of a highly fluorinated polymeric halocarbon having a diameter less than 1 micron, and a plas ticizer for the said polyisobutylene which is an ester having at least carbon atoms.

13. A flexible explosive sheet composition which comprises between 87 and 89% by weight of a crystalline high explosive in a fine particulate form with a bimodal particle size distribution which is bound in a visco-elastic binder comprising polyisobutylene and a plasticizer for said polyisobutylene and polytetrafluoroethylene in the form of extremely fine solid particles having a diameter less than 1 micron, said polyisobutylene and plasticizer therefor constituting between 10.5 and 11% by weight of the composition and said polytetrafluoroethylene constituting 1 to 1.5% by weight of the composition.

14. In a process for the manufacture of an explosive sheet composition the steps of intermixing a composition containing to by weight of a crystalline high explosive in a visco-elastic binder containing between 3 and 25 by weight of a highly fluorinated polymeric halocarbon as extremely fine particles made by a suspension process, and repeatedly passing the mixed composition through differential rollers having a separation less than 0.01 inch, whereby the high explosive is ground into fine particles having a bimodal size distribution and the halocarbon is dispersed throughout binder in the form of solid particles having a diameter less than 1 micron.

15. A process for the manufacture of an explosive sheet which comprises the process according to claim 14 followed by a final rolling of the composition between rollers separated by the desired thickness of the explosive sheet.

References Cited by the Examiner UNITED STATES PATENTS 2,999,743 9/1961 Breza et a1. 14919 3,102,833 9/1963 Schulz 14919 X. 3,117,044 1/1964 Sauer 149-49 3,138,501 6/1964 Wright 14919 X 3,173,817 3/1965 Wright 149-19 X CARL D. QUARFORTH, Primary Examiner.

BENJAMIN R. PADGETT, Examiner. 

1. A FLEXIBLE EXPLOSIVE SHEET COMPOSITION WHICH COMPRISES BETWEEN 85 AND 90% BY WEIGHT OF A CRYSTALLINE HIGH EXPLOSIVE IN A FINE PARTICULATE FORM WITH A BIMODAL PARTICLE SIZE DISTRIBUTION WHICH IS BOUND IN A VISCO-ELASTIC BINDER OF WHICH BETWEEN 3 AND 25% BY WEIGHT CONSISTS OF EXTERMELY FINE SOLID PARTICLES OF A HIGHLY FLUORINATED POLYMERIC HALOCARBON HAVING A DIAMETER LESS THAN 1 MICRON THROUGH SAID BINDER. 